Hey mate,
Usually i'd say no, but it's hard to tell if you don't give us the power consumption of the amp, or at least the WRMS output rating (from which the power consumption can be worked out)
Do you have a skyline? skylines usually have good charging systems. If you only listen to the stereo while the car is running i wouldn't worry about it, unless your stereo system is churning >5000WRMS then you'd probably want a larger alternator lol.
If you do listen to the stereo a lot while the car is off, and you've found that it's flattening the battery, you can do two things.
1. Install a second deep cycle battery in your car. (deep cycle batteries are very good at discharging most of their stored charge without lowering voltage, unlike regular car batteries which are designed for short bursts of great power) You will also need to install a charging manager to ensure that your main battery charges first, then when it is full it will begin to charge your second battery.
I wouldn't worry about that though...too much work, also only really neccesary if your stereo is churning stacks of power.
2. Install a hybrid battery in your car, these batteries are not quite as effective as two batteries, but are adept at both providing large surges of power to start your car even after being considerably drained. (from your car stereo say) This basically means you can use headlights/stereo/interior lights exc much longer before draining the battery beyond the point of starting your car.
Either way, i don't think your 2channel amp and monoblock amp will consume enough power to worry about, unless you leave it going while the car is off for hours at a time. If so i'd reccomend one of the above options. Or just turn your car back on for 10mins every hour or so
Hope that helps!